Micrel
MIC2172/3172
By using the MIC3172, U1 and Q1 shown in figure 5 can
be eliminated, reducing the total components count.
Synchronizing the MIC2172
Using several unsynchronized switching regulators in the
same circuit will cause beat frequencies to appear on the
inputs and outputs. These beat frequencies can be very
low making them difficult to filter.
Micrel’s MIC2172 can be synchronized to a single
master frequency avoiding the possibility of undesirable
beat frequencies in multiple regulator circuits. The
master frequency can be an external oscillator or a
designated master MIC2172. The master frequency
should be 1.05 to 1.20 times the slave’s 100kHz nominal
frequency to guarantee synchronization.
Application Information
Using the MIC3172 Enable Control (New Designs)
For new designs requiring enable/shutdown control,
connect EN to a TTL or CMOS control signal (figure 3).
The very low driver current requirement ensures
compatibility regardless of the driver or gate used.
Figure 3. MIC3172 TTL Enable/Shutdown
Using the MIC3172 in LT1172 Applications
The MIC3172 can be used in most original LT1172
applications
by
adapting
the
MIC3172’s
enable/shutdown feature to the existing LT1172 circuit.
Unlike the LT1172 which can be shutdown by reducing
the voltage on pin 2 (V
C
) below 0.15V, the MIC3172 has
a dedicated enable/shutdown pin. To replace the
LT1172 with the MIC3172, determine if the LT1172’s
shutdown feature is used.
Circuits without Shutdown
If the shutdown feature is not being used, connect EN to
V
IN
to continuously enable the MIC3172 or use an
MIC2172 with SYNC open (figure 4).
Figure 6. Master/Slave Synchronization
Figure 4. MIC2172/3172 Always Enabled
Circuits with Shutdown
If shutdown was used in the original LT1172 application,
connect EN to a logic gate that produces a TTL logic-
level output signal that matches the shutdown signal.
The MIC3172 will be enabled by a logic-high input and
shutdown with a logic-low input (figure 5). The actual
components performing the functions of U1 and Q1 may
vary according to the original application.
Figure 6 shows a typical application where several
MIC2172s operate from the same supply voltage. U1’s
oscillator frequency is increased above U2’s and U3’s by
connecting a resistor from SYNC to ground. U2-SYNC
and U3-SYNC are capacitively coupled to the master’s
output (V
SW
). The slaves lock to the negative (falling
edge) of U1’s output waveform.
Figure 7. External Synchronization
Figure 5. Adapting to the LT1172 Socket
Care must be exercised to insure that the master
MIC2172 is always operating in continuous mode.
Figure 7 shows how one or more MIC2172s can be
11
M9999-041806
(408) 955-1690
April 2006
MICREL [ MICREL SEMICONDUCTOR ]
